Dual band slot antenna



Oct. 18, 1960 .1. E. HOWELL ETAL 2,957,172

DUAL BAND SLOT ANTENNA Filed Sept. 5, 195a Jumes E. Howell Thomas A. Barr,

. I 4N ENT s.

.S ATTORNEYS.

2,951,112 4 DUAL BAND sLor ANTENNA James Howell, 718 E. Holmes St., and Thomas A. Barr, 4618 Panorama Drive SE., both of Huntsville, Ala.

Filed Sept. s, 1958, Ser- No. 759,372 (Cl. 343-446 (Granted under Title as, US. Code 1952 sec. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon.

Our invention relates to slot antennas for transmitting radio frequency waves and more particularly to such an antenna disposed for simultaneous operation on multiple frequencies.

A conventional slot antenna includes a sheet of conducting material with a slot therein. The slot is usually rectangular with one dimension of the rectangle being equal to one-half wave length and the other dimension being small in comparison therewith. Such a sheet radiates in both directions normal to the sheet but the antenna is provided with a metallic box disposed on one side of the slot to limit radiation to the opposite side of the sheet. The depth of the slot is substantially a quarter wave length.

In applications such as missiles in which weight and size are critical, the length of the slot and the depth of the box are reduced and the reduced antenna may be loaded to function at the same frequency by connecting a capacitive device such as a stub or capacitor between the long edges of the slot. Such expedients are well known in the art.

In carrying out our invention, one embodiment of an antenna disposed for simultaneous operation at low and high frequencies is provided with a sheet of conductive material with a rectangular slot therethrough having the long edges thereof shorter than the wave length of the higher frequency, a metallic box in electrical contact with the portion of the sheet enclosing the slot and a pair of capacitive reactance devices respectively connected between the long edges substantially at the center and one of the quarter positions thereof.

The capacitive reactance devices are tuned for respective resonance of the entire slot at the low frequency and of the half slot adjacent the quarter position at the high frequency.

In another embodiment of the device, a filter is disposed in the box for connection between the center position and a receiver with a high sensitivity for reception of weak signals transmitted to the missile. The filter is provided with an impedance for matched relation between the low frequency circuit and the receiver.

For more complete understanding, reference is directed to the following description and accompanying drawings, in which:

Fig. 1 is an elevation including the open face of a slot antenna;

Fig. 2 is a view along line 2-2 of Fig. 1;

Fig. 3 is a view along line 3-3 of Fig. 1; and,

Fig. 4 is a view along line 4-4 of Fig. 1.

Accordingly, an antenna 12 is disposed for simultaneous operation in a pair of modes of predetermined frequencies in which f =rf where r=2. Antenna 12 is provided with a conducting plate 14, having a rectangular slot therein, and a metallic box 16 secured to a face of plate 14 contiguously to enclose the slot and form a cavity 2,957,172 Patented Oct. 18 1960 for resonance. Box 16 includes long sides 18 and short sides 20 and 22,: respectively corresponding to the long and short opposing edges of the slot, with side 18 substantially equal to-the wave length of frequency f A pair of capacitive reaction devices 24 and 26 are electrically connected between respective sides 18, and a I pair of terminals 28 and 30 are connected to the back 31 of box 16, in positions substantially at the mid point, one of the quarter points thereof.

Circuits of frequencies 1; and 1; include coaxial cables 32 and 34 with outer conductors respectively connected to terminals 28 and 30 and inner conductors insulated therefrom, and respectively connected to corresponding ends 27 of loops 36 and 38. The opposite ends 29 of the loops are respectively connected to terminals 28 and 30 to provide magnetic coupling of the cables with antenna 12.

Devices 24 and 26 are provided with capacitive reactances adjustable for resonance of the entire cavity within box 16 and the half cavity adjoining device 26 respectively at frequencies f and f Capacitive reactance varies inversely with the frequency, and with ratio r=2, the capacitive reactance of device 24 is low at frequency f to present a virtual short circuit thereto, and restrict f resonance to the half slot adjacent device 26. At frequency h, the capacitive reactance of device 26 is high to present negligible opposition to resonance of the entire cavity.

Similarly, a third capacitive reactance device disposed at a position 44 substantially midway between device 26 and side 22 and spaced in even harmonic relation with devices 24 and 26. The third device would be disposed for adjustment at a frequency f;, in which f =rf The f device would have high capacitive reactance at frequency 212;, to present little opposition to resonance of the half cavity and the capacitive reactance of device 26 would appear as a short circuit to cause the quarter cavity adjacent the 3 device to resonate at frequency i The frequencies of the terminal connections of the devices are in inverse relation with the even harmonic relation.

An additional coaxial conductor for f device would be. similarly magnetically coupled to antenna 12.

Coupling between the circuits is very loose and in one example of the device the isolation between the f and f circuits was found to be 30 decibels.

In the application shown, box 16 includes an aperture 40 with a filter 42 therein connected to the inner conductor of coaxial cable 32 and provided with an impedance for matched relation between the low frequency portion of antenna 12 and a receiver with high sensitivity. Connection to the receiver is indicated at 46. Filter 42 is further required to be highly efficient for operation of the antenna in atmospheres of low signal density.

While the foregoing is a description of the preferred embodiment, the following claims are intended to include those modifications and variations that are Within the spirit and scope of my invention.

We claim:

1. An antenna comprising a conducting plate provided with a pair of opposing faces and an intermediate slot with pairs of opposing long and short edges joining said faces; capacitive reactance devices connected between said long edges and disposed in positions spaced in parallel substantially even harmonic relation between said short edges; and loops corresponding to said capacitive reactance devices with pairs of conductors in respective series relation with said loops secured to said plate to provide transmission of energy at said faces and simultaneous energizations of said conductors at respective frequencies in inverse proportion with the spaced relation.

2. An antenna as in claim 1 with said long edges equal to substantially half of the wave length of the least of the V" frequencies.

3. An antenna as in claim 2 with said loops correspondingly disposed between said short edges to provide minimum transfer: of energy between the respective con 7 ductors for loose coupling therebetween. a

'4. An antenna'as in claim'3 with a metallic box provided with a pair of sides secured in conductive relation 7 with one of said faces contiguous to said long edges and a back secured to said sidcs to augment the transmission 7 of energy at the other of said faces; said devices being secured .to said sides for the respective connections.

5. An antenna as in claimt3 with a pair of said devices spaced in the evenharmonic relation; a terminal Secured nection of said antenna; and a filter disposed in a cavity to said box and insulated therefrom for external con- References Cited the iile oi this patent v v UNITED STATES PATENTS; I

2,573,460 Lindenblad Oct. 3o;-1951 2,586,895 'Willoughby v a, Feb. 26, 1952 2,794,184 7 Kolar et l. p May28, 1957 

